In various fields including chemistry, electric power, iron and steel, and industrial waste disposal, there has been used a ceramic porous honeycomb structure excellent in thermal resistance, corrosion resistance, and mechanical strength as a dust collecting or water treatment filter used for environmental measures such as pollution control, product recovery from high temperature gas, or the like. For example, as a dust collecting filter used at high temperature in a corrosive gas atmosphere, such as a diesel particulate filter (hereinbelow sometimes referred to as a “DPF”) for trapping particulate matter (hereinbelow sometimes referred to as “PM”) discharged from a diesel engine, a ceramic porous honeycomb structure has been used. The honeycomb structure is provided with porous partition walls separating and forming a plurality of cells functioning as fluid passages, and predetermined cells each having an opening on one end portion and a plugged portion on the other end portion and the residual cells each having a plugged portion on one end portion and an opening on the other end portion, where a fluid (exhaust gas) flowing into the structure from one end portion, where each of the predetermined cells has an opening, passes through the partition walls and flows into the residual cells as a penetrating fluid, which is discharged from the other end portion, where each of the residual cells has an opening, thereby trapping and removing PM in exhaust gas.
A process for producing the aforementioned ceramic porous honeycomb structure has, for example, a mixing step where forming raw materials including a ceramic raw material are mixed to obtain a forming blended material, a kneading step where the forming blended material is kneaded to obtain kneaded clay, a forming step where the clay is formed into a honeycomb shape to obtain a honeycomb formed article, and a firing step where the honeycomb formed article is fired to obtain a honeycomb structure. The honeycomb structure obtained in such a manner has a problem of easily deteriorating filtration performance (trapping efficiency) because an internal defect such as a coarse pore and a crack tends to generate. In particular, in recent years, since thinning of partition walls of a honeycomb structure has rapidly been proceeding, the internal defect such as a coarse pore generates more easily, which is becoming a serious problem.
As a cause of generating such an internal defect, presence of a coarse aggregate formed in such a manner that particulates in a framework particle raw material aggregate. There has been a proposal to inhibit an internal defect from generating by removing such a coarse aggregate or by inhibiting an aggregate from generating (see, e.g., Patent Documents 1, 2).    Patent Document 1: WO2001/058827    Patent Document 2: WO2005/018893